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The FDA approves Bristol-Myers Squibb's Opdivo, pitting it against Merck's Keytruda that was approved earlier this year.

That's not very surprising given the strong clinical trial data the cancer drug has posted so far. In a trial of 120 patients without any other options, 32% of patients responded to the drug, and about a third of those experienced a response for more than six months.

Source: Bristol-Myers Squibb.

Even the fact that it was approved three months before the FDA's PDUFA goal isn't that big of a surprise. Lately, the oncology division has not taken its fully allotted time to review drugs, especially those looking for accelerated approval based on limited-but-promising data. Cancer is the one area where "breakthrough therapy" designation has actually lived up to its potential to speed patient access to high-impact drugs.

The surprise is comingThe big unknown here is how Opdivo will compete with Merck's (NYSE:MRK) Keytruda, which was approved earlier this year. Both drugs target the same pathway, PD-1, which is an internal block that tells the immune system not to attack human cells. Since tumors are made up of human cells -- albeit ones that are growing uncontrollably -- the PD-1 checkpoint blocks the ability of the immune system to attack the tumor cells. Disrupting that block allows the immune system to attack the tumor.

Figuring out whether Opdivo or Keytruda will win is complicated by the limited clinical trial data, despite both drugs being approved. Both drugs were given accelerated approval, meaning they show promise in treating a disease with a high medical need -- in this case, melanoma patients that have exhausted other options -- but have to prove their worth in a later clinical trials.

Keytruda managed to shrink tumors in about 24% of patients. While that's less than the 32% response rate for Opdivo, it's really hard to make a comparison between trials since they may have enrolled patients with different severities of their diseases.

You can make an educated guess about the enrolled patients between two clinical trials if the comparator groups have similar results. Unfortunately, since this is a last-line-of-defense treatment, there isn't an active comparator drug, and using placebo would be unethical since the response rate should be 0%.

Keytruda is more convenient, being dosed every three weeks versus every two weeks for Opdivo, but when you're dying of cancer, convenience would seem to be a minor consideration. Doctors might even prefer Opdivo because it provides more frequent check-ins by the patients.

Other tumors key Neither drug is going to reach its full potential as a last-line-of-defense treatment for advance melanoma; the market is just too small. Fortunately, since the drugs regulate the immune system, it's possible for the drugs to treat other types of cancer.

Data from small phase 1 trials presented at American Society of Hematology earlier this month suggest that both drugs work well in patients with Hodgkin lymphoma. Opdivo produced an overall response rate of 87%, with 17% of patients having a complete response. Patients taking Keytruda didn't respond quite as well -- 66% response rate -- but 21% had a complete response, besting Opdivo. Like the melanoma trials, making comparisons between the small trials is difficult because the baseline characteristics of the patients may not be the same.

The best hint as to which drug might come out on top will come from ongoing phase 3 trials in lung cancer being run by both Merck and Bristol-Myers Squibb. While the trials won't compare the drugs directly, their larger size and the fact that they have active comparators should make rudimentary comparisons possible.

We won't have to wait long; interim data from Bristol's trial testing Opdivo is expected in the early part of next year.